# Re: Teach SELECT DISTINCT first!

From: Alan <alan_at_erols.com>
Date: Wed, 28 Apr 2004 14:12:47 -0400
Message-ID: <c6os70\$eobhr\$1_at_ID-114862.news.uni-berlin.de>

"Laconic2" <laconic2_at_comcast.net> wrote in message news:ttadne9jwZQY7BPdRVn-tw_at_comcast.com...
> Sorry, but it isn't just a sophism, although it looks like one, on the
> surface.
>
> The fact is that representing relations (or, for that matter sets) inside
a
> computer is a much bigger challenge than most people think. The reason is
> that the bits in memory have a natural order, according to the addressing
> scheme of whatever memory they are in. Representing a set, without
> implying order, necessarily implies imposing an artificial order on the
> representation, that is not homomorphic to the thing being represented.
>
> That is why I asked, a while ago whether a pizza with pepperoni and onion
> is the same thing as a pizza with onion and pepperoni. If the list of
> toppings truly represents a set, then, in order to let a PICK record
> represent a relation,
> we have to forbid duplicates, even when the MV fields contain the same
> values, but in a different order.
>
> But if the list of toppings represent a list (NOT a set), then two lists
> made up of the same values, but in different orders, are different
lists,
> and therefore don't duplicate each other.
>
> Telling whether two sets are equivalent, without doing any sorting, is
> nearly impossible. That is why, IMO, Codd imposed the "no compound
> columns" rule on 1NF, even though the rules of relational algebra don't
> require that.
>
> And it's the "no compound columns" rule, not the "no duplicate rows"
rule
> that provoked the long discussion of whether 1NF was a good idea or a bad
> idea. Or a good idea in 1970, but a bad idea in 2004. And that's not
> sophism.
>

With the talk of "homomorphic" and all, this should probably be considered a "Sapphism", not a "sophism" Received on Wed Apr 28 2004 - 20:12:47 CEST

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